Bone anchored hearing aids are essential for the rehabilitation of patients suffering from some specific type of hearing losses for which traditional hearing aids are insufficient. This type of device consists of an external hearing aid with a vibrating transducer which is connected via a connector to a skin penetrating abutment mounted on a fixture anchored in the skull bone. The abutment can be mounted on the fixture with a small screw going through the centre of the abutment and into a threaded hole in the fixture. It is important that the coupling between the hearing aid and the abutment is sufficiently firm to avoid poor transmission of the vibrations but it is also important that the coupling is not too firm since it is also important that the hearing aid falls off in case of a sudden impact to avoid that the skull bone anchoring is damaged. The patient takes on and off the hearing aid daily so wear and tear durability of the coupling is important.
One type of an interconnecting coupling between the hearing aid and the abutment is described in U.S. Pat No. 5,735,790. A drawback with this design is that it is difficult to change the coupling part on the hearing aid when this is worn out. Significant costs and problems for the patients are caused by the fact that the device needs to be sent in to a special hearing aid repair centre to change the coupling part on the hearing aid. Another drawback with that design is that it is difficult to get a constant coupling force during the life time of the coupling since there is no reliable metal spring which determines the coupling force and which is more stable during long term use. Since the invention described in U.S. Pat. No. 5,735,790 does not include any easy removable coupling spring it is also not easy to modify the coupling force for individual fitting of a suitable coupling force for different patients. It is for example desirable to have a lower coupling force on a young child who has a softer skull bone that on an adult patient. The invention described in U.S. Pat. No. 5,735,790 includes a flexible coupling part going inside of the abutment which is a design which can limit the size of the conical entering of the coupling which is necessary to make it easy for especially elderly patients to find the way into the coupling when the hearing aid is connected to the abutment. The invention described in SE 89032718 had flexible coupling arms which extended along the centre axis of the coupling over a ball shaped part representing a significant portion of the abutment length which increased the risk for the flexible coupling to interfere with the skin around the other end on the abutment. The invention described in SE 89032718 had no separate spring arrangement so it is difficult to change the coupling part on the hearing aid and the invention therefore also includes several of the drawbacks described for in U.S. Pat. No. 5,735,790. Both of these inventions also have flexible arms in the axial direction and where the flexible arms include slots in the axial direction where the abutment or something else can get into the slots and damage the spring arrangement. An arrangement intended to hinder such damage is likely to increase the collection of dirt in the coupling. The invention described in U.S. Pat. No. 4,498,461 is a bayonet coupling which is a design with several drawbacks since it includes wear and tear parts mounted into the abutment on patient instead of mounted on the easy removable hearing aid part of the system. This design also results in a very varying coupling force during the life time of the device. A design like the one described in SE0102207 has several drawbacks since it includes magnetic components which are not compatible with MRI equipment used at hospitals and a small magnetic circuit can not generate the force needed to keep the hearing aid properly in place. A connector system can include a connector mounted directly on the hearing aid or a connector on any kind of separatable interconnection unit which can be connected in between the connector on the hearing aid and the abutment.